This invention relates to a process for synthesizing polyol fatty acids polyesters in which unreacted lower alkyl ester is recovered from the reaction mixture and recycled for use in the polyol fatty acid polyester synthesis. More particularly, this invention relates to such a process wherein good product quality of polyester synthesized with the recycle ester is maintained by minimizing alkyl ester degradation reactions such as oxidation, hydrolysis, pyrolysis, and saponification.
The food industry has recently focused attention on polyol polyesters for use as low-calorie fats in food products. Triglycerides (triacylglycerols) constitute about 90% of the total fat consumed in the average diet. One method by which the caloric value of edible fat can be lowered is to decrease the amount of triglycerides that is consumed, since the usual edible triglyceride fats are almost completely absorbed in the human system (see Lipids, 2, H. J. Deuel, Interscience Publishers, Inc., New York, 1955, page 215). Low calorie fats which can replace triglycerides are described in Mattson, et al., U.S. Pat. No. 3,600,186. Mattson, et al. disclose low calorie, fat-containing food compositions in which at least a portion of the triglyceride content is replaced with a polyol fatty acid polyester having at least four fatty acid ester groups, with each fatty acid having from eight to twenty-two carbon atoms.
Rizzi and Taylor, U.S. Pat. No. 3,963,699, disclose a solvent-free transesterification process in which a mixture of polyol (such as sucrose), a fatty acid lower alkyl ester (such as a fatty acid methyl ester), an alkali metal fatty acid soap (emulsifier), and a basic catalyst is heated to form a homogenous melt. Excess fatty acid lower alkyl ester is added to the melt to form the higher polyol fatty acid polyesters. The polyesters are then separated from the reaction mixture using various separation procedures; distillation or solvent extraction are preferred.
Volpenhein, U.S. Pat. Nos. 4,517,360 and 4,518,772, discloses a solvent-free transesterification process in which a mixture of polyol (such as sucrose), fatty acid ester selected from the group consisting of methyl esters, 2-methoxy ethyl esters, and benzyl esters, an alkali metal fatty acid soap, and a basic catalyst is heated to form a homogenous melt, to which is added excess fatty acid ester to form the higher polyol fatty acid polyesters. The polyesters are then separated from the reaction mixture using various separation procedures; distillation, water washing, conventional refining techniques or solvent extraction are preferred.
Bossier (III) U.S. Pat. No. 4,334,061, discloses a process in which a mixture of polyol, fatty acid ester, alkali metal fatty acid soap, and basic catalyst is heated to form a homogenous melt, to which is added excess fatty acid ester to form the polyol fatty acid polyesters. The polyesters are then recovered by contacting the crude reaction product with an aqueous washing medium while maintaining the resulting mixture at a pH of from 7 to about 12, in the presence of an emulsion decreasing organic solvent. The alkali metal fatty acid soaps and the color-forming bodies are dissolved in the aqueous phase. The polyol fatty acid polyester is recovered from the organic phase by solvent extraction to remove excess fatty acid lower alkyl esters and steam stripping to remove trace amounts of residual fatty acid lower alkyl esters and solvent.
Virtually all of the polyol fatty acid polyester synthesis processes require that the polyol fatty acid polyester be separated from a reaction mixture comprising products, by-products, and unreacted ingredients. Additionally, many polyol polyester synthesis processes require the use of excess lower alkyl ester, in particular excess methyl ester, so that a significant amount of unreacted lower alkyl ester is contained in the reaction mixture from which the polyol polyester product is recovered. The polyol fatty acid polyester synthesis would therefore be more economically efficient if the excess methyl esters could be reused in the polyol fatty acid polyester synthesis. However, because significant degradation of the lower alkyl esters can occur in conventional processing steps employed to separate and purify the polyol polyester product or in separation of the unreacted lower alkyl ester from the reaction mixture, reuse of the degraded lower alkyl ester can result in the synthesis of inferior polyol polyester product. Consequently, there remains a need to develop a process which can recycle the excess lower alkyl ester from a polyol fatty acid polyester synthesis without adversely affecting product quality of polyesters synthesized from the recycled ester.
Accordingly, it is an object of this invention to obviate problems encountered in the prior art and provide improved processes for synthesis of polyol fatty acid polyesters.
It is another object of this invention to minimize the side reactions which degrade lower alkyl esters during such processes to allow the recycle of excess lower alkyl ester without adversely impacting the quality of polyol fatty acid polyester produced therefrom.
It is yet another object of this invention to provide a novel process for the production of polyol fatty acid polyesters, which process recycles unreacted ingredients and improves the economics of the polyol synthesis.
It is a related object of this invention to provide a novel process for the production of polyol fatty acid polyesters, which process eliminates the need to dispose of significant amounts of unused excess reactants.
In accordance with one aspect of the present invention, there is provided a novel process for synthesizing polyol fatty acid polyester comprising the steps of reacting lower alkyl ester and polyol, partially esterified polyol or mixtures thereof to esterify hydroxyl groups thereof and form polyol fatty acid polyester comprising partially and/or fully esterified polyol in admixture with unreacted lower alkyl ester; separating at least a portion of the unreacted lower alkyl ester from the polyol fatty acid polyester; and recycling the separated unreacted lower alkyl ester for further reaction with polyol or partially esterified polyol, wherein the recycled lower alkyl ester is substantially free of lower alkyl ester degradation reaction products.
In accordance with another aspect of the present invention there is provided a novel transesterification process for synthesizing polyol fatty acid polyester comprising the steps of heating a mixture of polyol, fatty acid lower alkyl ester, basic reaction catalyst, and optionally an alkali metal fatty acid soap to form a reaction mixture; subsequently adding to the reaction mixture excess fatty acid lower alkyl ester; reacting a portion of said fatty acid lower alkyl ester with polyol to obtain a product mixture; separating unreacted fatty acid lower alkyl ester from the product mixture; and recycling the separated unreacted fatty acid lower alkyl ester for further reaction, wherein the recycled lower alkyl ester is substantially free of degradation reaction products.
It has been found that unreacted lower alkyl esters can be recovered from the product mixture of product, by-products and unreacted ingredients, and recycled for use in the polyol fatty acid polyester synthesis with no adverse impact on the polyol fatty acid polyester reaction or on the quality of the polyester produced. Potential degradation reactions, such as oxidation, hydrolysis, pyrolysis, and saponification, are minimized so as to recycle directly back to the synthesis reactor the unreacted lower alkyl ester which is substantially free of degradation reaction products. The recycling of unreacted lower alkyl esters according to the invention improves the economics of the synthesis reaction, since separated unreacted fatty acid lower alkyl esters which contain high levels of degradation product would need to be further processed to remove substantial amounts of the degradation products from the ester recycle, or otherwise would need to be discarded, either of which can be expensive.
Additionally, it has been found that the same basic compounds which are used to catalyze the polyol fatty acid polyester synthesis can also be used to neutralize fatty acids in the recycled ester, thereby further improving the economical aspects of the synthesis processes employing ester recycle.
These and additional objects and advantages will be more filly apparent in view of the following detailed description.
The present invention encompasses processes for recycling lower alkyl esters in the synthesis of polyol fatty acid polyesters. Lower alkyl ester recycling can be used in conjunction with any polyol fatty acid polyester synthesis method which utilizes lower alkyl esters. Such processes are disclosed in U.S. Pat. Nos. 3,963,699; 4,517,360; 4,518,772; 4,806,632 and 5,491,226, incorporated herein by reference. One suitable polyol fatty acid polyester synthesis process is a solvent-free transesterification reaction which can be performed in two steps. In the first step of the transesterification synthesis process, polyol, fatty acid lower alkyl ester, basic reaction catalyst, and optionally soap are combined to form a heterogeneous mixture.
As used herein, the term xe2x80x9cpolyolxe2x80x9d is intended to include any aliphatic or aromatic compound containing at least two free hydroxyl groups. Suitable polyols can be selected from the following classes: saturated and unsaturated straight and branch chain linear aliphatics; saturated and unsaturated cyclic aliphatics, including heterocyclic aliphatics; or mononuclear or polynuclear aromatics, including heterocyclic aromatics. Carbohydrates and non-toxic glycols are preferred polyols. Monosaccharides suitable for use herein include, for example, mannose, glucose, galactose, arabinose, xylose, ribose, apiose, rhamnose, psicose, fructose, sorbose, tagatose, ribulose, xylulose, and erythrulose. Oligosaccharides suitable for use herein include, for example, maltose, kojibiose, nigerose, cellobiose, lactose, melibiose, gentiobiose, turanose, rutinose, trehalose, sucrose and raffinose. Polysaccharides suitable for use herein include, for example, amylose, glycogen, cellulose, chitin, inulin, agarose, zylans, mannan and galactans. Although sugar alcohols are not carbohydrates in a strict sense, the naturally occurring sugar alcohols are so closely related to the carbohydrates that they are also preferred for use herein. Natural sugar alcohols which are suitable for use herein are sorbitol, mannitol, and galactitol. Particularly preferred classes of materials suitable for use herein include the monosaccharides, the disaccharides and sugar alcohols. Preferred polyols include glucose, fructose, glycerol, polyglycerols, sucrose, zylotol, sorbitol, alkoxylated glycerines, alkoxylated polyglycerols, and sugar ethers; particularly preferred is sucrose.
As used herein, the term xe2x80x9cpolyol fatty acid polyestersxe2x80x9d is intended to include fatty acid esters of polyols, in which one or more of the hydroxyl groups are replaced with esters of fatty acids. Preferred polyol fatty acid polyesters are those wherein at least half of the hydroxyl groups have been replaced with esters of fatty acids. Particularly preferred are sucrose polyesters with at least five ester linkages per sucrose molecule, in which the fatty acid chains have from about eight to about twenty-four carbon atoms. As used herein, the term xe2x80x9clower alkyl esterxe2x80x9d is intended to include fatty acid esters of lower alkyl alcohols, in which the hydroxyl groups are replaced with esters of fatty acids. Suitable lower alkyl alcohols include C1-C6 mono-alcohols. Especially preferred lower alkyl esters are methyl esters.
Suitable fatty acid esters can be derived from either saturated or unsaturated fatty acids. Suitable preferred saturated fatty acids include, for example, capric, lauric, palmitic, stearic, behenic, isomyristic, isomargaric, myristic, caprylic, and anteisoarachadic. Suitable preferred unsaturated fatty acids include, for example, maleic, linoleic, licanic, oleic, linolenic, erythrogenic acids. In a preferred embodiment of the invention, the fatty acid chains have from about two to about twenty-four carbon atoms. Hydrogenated or unhydrogenated lower alkyl esters obtained from soybean oil, palm kernel oil, coconut oil, sunflower oil, safflower oil, corn oil, cottonseed oil, peanut oil, canola oil, high erucic acid rapeseed oil, and mixtures thereof are preferred.
In the present processes, polyol fatty acid polyester is synthesized and unreacted lower alkyl ester is recycled. In particular, a polyol fatty acid polyester is synthesized by a process comprising the steps of (a) reacting lower alkyl ester with polyol, partially esterified polyol, or mixtures thereof to esterify hydroxyl groups thereof and form polyol fatty acid polyester, the polyol fatty acid polyester comprising partially esterified polyol, fully esterified polyol, or mixtures thereof, and being in admixture with unreacted lower alkyl ester, (b) separating at least a portion of the unreacted lower alkyl ester from the polyol fatty acid polyester, and (c) recycling the separated unreacted lower alkyl ester for further reaction with polyol or partially esterified polyol, wherein the lower alkyl ester which is recycled for further reaction is substantially free of lower alkyl ester degradation reaction products. As used herein, xe2x80x9csubstantially freexe2x80x9d of lower alkyl degradation reaction products means that the use of the recycled lower alkyl ester will not adversely effect the quality of the polyol polyester product formed from the recycled lower alkyl ester. Degradation reaction products of the lower alkyl ester will be apparent to those skilled in the art and comprise products of oxidation, hydrolysis, pyrolysis, saponification and the like. These degradation reaction products should be minimized or eliminated to permit direct recycling of the lower alkyl ester to the polyol polyester reaction, without resorting to extra cleanup or purification steps for the recycled esters.